The starting point of the present invention is disclosed in U.S. Pat. No. 4,644,341 to Warner, which is incorporated herein by reference. That patent generally describes a photosensor utilizing a contrast indicating arrangement and LED bargraph display to give a visual indication of the light level returned to the photosensor as detected in the light and dark states. The difference between the maximum (light) reading on the bargraph display and the minimum (dark) reading on the bargraph display is termed the "contrast differential."
The determination of the contrast differential is important to a photosensor's successful performance, as the proper adjustment of the photosensor during operation is largely dependent on this determination. In particular, during sensing tasks where the difference between the "light" and "dark" states is very small, the precise adjustment of the photosensor is critical. For example, consider the situation where the presence of metal washers on screws are being detected by the photosensor. The difference between the returned light levels from the presence of the washers as compared to the absence of the washers is very small. Thus, it is desirable to adjust the operation of the photosensor (by manipulation of the photosensor's gain or bias controls) so as to "increase" the difference in the returned light levels between the presence and absence of the washers. And, it is desirable that this adjustment take place during ongoing sensing tasks, such that shutdown of the sensing system or other such industrial application (which can be very costly) due to faulty photosensor performance is avoided.
The invention disclosed in the aforementioned U.S. Pat. No. 4,644,341 was designed to facilitate proper adjustment of a photosensor's performance during operation. To accomplish this, a pulse modulator circuit is connected to the sensor's light source (typically an LED). Reflected light is received by a light sensor (typically a photo diode or photo transistor). The received light is amplified and then demodulated by a peak detector circuit. After filtering, the received light signal is impressed on a DC amplifier circuit, which outputs an analog DC signal. Preferably this analog DC signal is directly proportional to the intensity of the received light.
The analog DC signal is then input to the LED bargraph display, thereby giving an indication of the available light contrast to the user. If adjustments to the sensor need to be made, the gain and bias of the DC amplifier circuit (which are typically variable resistors) are exposed on the outside of the photosensor for manual adjustment. Thus, by examining the intensity of the received light (as embodied by the analog DC signal) on the bargraph display, the user is able to adjust the photosensor in such a way as to achieve maximum operating performance.
While the foregoing invention provided a marked improvement in the reliability and operation of photosensors, it has limitations in the area of extremely low contrast sensing tasks. Every photosensor has a "saturation point;" that is, the point at which any further increase in received light by the light sensor will not result in any further increase in the magnitude of the internal signal. This would be apparent from the photosensor's bargraph display, in that a change in an object sensing task will not result in an increase in the signal level displayed by the bargraph.
An example may prove illustrative. Consider again the situation where the sensing task is to detect the presence or absence of metal washers on corresponding metal screws. If the background (i.e., the screw) is reflecting enough light to reach the photosensor's saturation point, the presence of the washer will not produce a change in the signal shown on the bargraph display. Of course, in such a situation, the photosensor is not performing its sensing task.
The undesirable situation described above is referred to as "dark state saturation." One of the goals of the present invention, therefore, is to provide a photosensor which substantially avoids the dark state saturation condition and enhances background reflection suppression.
Conventional techniques for accomplishing these goals have a number of limitations. For example, in the above-described photosensor, the amplifier gain must be reduced to operate the photosensor in bright received light conditions. Reducing the photosensor's amplifier gain, however, necessarily minimizes the sensor's ability to perform low contrast sensing tasks. Accordingly, the performance of the photosensor is compromised.
Other potential solutions in the prior art are also not acceptable. For example, it has been proposed to turn down the intensity of the light source, or to replace the color of the light source with one of weaker intensity. These solutions, however, are not reliable, as they are only estimations of how much the light source intensity must be decreased in order to avoid entering saturation. Random adjustment of the light source intensity may result in the photosensor still entering saturation if the adjustment is not enough, or will result in overcompensation, such that the ability of the photosensor to perform low contrast sensing tasks is lessened.
One type of prior art system that adjusts the intensity of the light source is disclosed in U.S. Pat. Nos. 5,281,810 and 5,336,882. In this system a microprocessor is used to control the repetition rate and amplitude level of the current and resulting light pulses of the LED light source. The amplitude and pulse rate are adjusted together in order to avoid overloading the LED light source. The microprocessor responds to a pulse rate selected by the operator, in accordance with his or her particular response time and range needs. Of course, it is understood that as the pulse rate increases, the response time of the sensor improves but its overall sensing range decreases. This type of system allows the user the flexibility to select the most appropriate repetition rate depending on the particular sensing task.
While the system disclosed in the above-mentioned U.S. Pat. Nos. 5,281,810 and 5,336,882 provides many important advances in the photosensor field, it has its drawbacks. To begin with, the system is rather complex, requiring a sophisticated microprocessor for operation. While this system uses a comparison algorithm to automatically adjust the gain of the amplifier in response to reflected signals that are below a threshold value or are approaching saturation, this system does not adjust the input light intensity in direct response to an approaching saturation condition. Instead, this system adjusts the gain of the amplifiers. The system in the above-referenced patents therefore does not disclose a photosensor which varies the transmitted light intensity in response to adjustment of the offset of the DC amplifier. While this prior art system does utilize an LCD display to transmit system information to the user, it does not disclose the use of a easy-to-understand visual indication of when the photosensor is operating within its intended dynamic range, as will be described below in accordance with the present invention.
Returning to the problem at hand, other solutions known in the prior art for preventing dark state suppression, such as backing the photosensor away from the object sensing field, or changing the diameter of the light source fiber optics, or reducing amplifier gain are also not suitable. These solutions will reduce the photosensor's ability to perform low contrast sensing tasks, and therefore compromise the sensor's performance ability.
Many of the above-proposed solutions are also not acceptable because photosensors typically do not have a fixed saturation point. Photosensors must be versatile enough to perform in a variety of sensing tasks, which therefore require the sensor to have the ability to detect numerous different changes in contrast. Any of the above-described adjustments, thus, will reduce or significantly degrade the photosensor's ability to perform difficult low contrast sensing tasks. The capability of the photosensor to adapt to a wide variety of contrasting light levels is also diminished, without constant "trial and error" adjustment. Also, because many photosensors are located such that the fiber-optics must be in a fixed position, the above-described adjustments will all detrimentally effect the response of the photosensor in some way.
Accordingly, there exists a need in the photosensor industry for a photosensor that can resolve low contrast sensing tasks over a wide range of light intensities, and still maintain proper operation during high reflected light levels. It is also desirable to develop a photosensor that may be adjusted to avoid reaching the saturation point without sacrificing amplifier gain. Further, there exists a need for a photosensor that provides the user with an indication of when the photosensor is either approaching saturation or operating under very low received light conditions, so that an appropriate performance adjustment may be made to keep the photosensor operating within dynamic operating range, thereby ensuring that contrast deviation response is maximized.